Fix for issue #11624

#11624
#6156
#2803
#11242
#10842

Plus others probably...

quadrilateral_angles assumed that some quadrilaterals were trapezoids without proving it. This annotates the sides as being parallel and adds a note saying as much to the question. It does not do this for isosceles quadrilaterals or parallelograms since working this out from the information given is a good part of the question.

It doesn't do this terribly generically however. For a given quadrilateral only a single set of lines can be marked as parallel and it will only look right is the lines are nearly vertical or nearly horizontal. I believe it is still an improvement however.

Sandcastle: http://sandcastle.khanacademy.org/pull/11724/